1. Field of the Invention
The present invention relates to a process for producing a semiconductor wafer with a front surface and a back surface, in which the semiconductor wafer is subjected to two-sided polishing.
2. The Prior Art
A device which is suitable for the double-side polishing of semiconductor wafers is described, for example, in U.S. Pat. No. 3,691,694. According to one embodiment of double-sided polishing which is described in EP 208 315 B1, semiconductor wafers are polished in carriers made from metal, which have suitably dimensioned, plastic-lined cutouts, between two rotating polishing plates, which are each covered with a polishing cloth.
Polishing of this type often leaves behind scratches and punctiform elevations, which are visible at a resolution of a few xcexcm, in the region of the edge of the semiconductor wafer.
It is an object of the present invention to provide a process for double-sided polishing of semiconductor wafers in which no such irregularities are formed in the region of the wafer edge.
The above object is achieved by the invention which relates to a process for producing a semiconductor wafer with a front surface and a back surface, in which the semiconductor wafer is subjected to two-sided polishing, which comprises the following temporal sequence of steps:
(a) producing a hydrophobic surface on the semiconductor wafer by treating the semiconductor wafer with an aqueous HF solution;
(b) simultaneous polishing of the front surface and the back surface of the semiconductor wafer with a surface which has been rendered hydrophobic, with an alkaline polishing abrasive being continuously supplied between two rotating upper and lower polishing plates, which are both covered with a polishing cloth, the pH of the polishing abrasive being from pH 8.5 to pH 12.5;
(c) after an intended polishing abrasion has been reached, supplying a stopping agent to the semiconductor wafer; and
(d) removing the semiconductor wafer from the polishing plates.
An essential feature of the invention is that the double-side polishing in accordance with step (b) is carried out with a semiconductor wafer which immediately beforehand has been treated with an aqueous HF solution and has thus acquired a hydrophobic surface. In this way, the surface of the semiconductor wafer remains hydrophobic from the start of polishing through to the end of polishing. It has been found that a surface condition of this nature is required, since this ensures that the formation of hydrogen bubbles, which inevitably occurs during the polishing, is restricted and that the bubbles adhere less strongly to the edge of the semiconductor wafer. It is assumed to be hydrogen bubbles which adhere to the surface of the edge of the semiconductor wafer which prevent uniform etching removal and are responsible for the occurrence of the abovementioned defects in the region of the edge of the semiconductor wafer.
The treatment with aqueous HF solution preferably takes place in a bath into which the semiconductor wafer is dipped on its own or together with other semiconductor wafers. Furthermore, it is also possible to spray the semiconductor wafer with the HF solution or to expose the semiconductor wafer to a moist HF atmosphere for a certain time. The desired hydrophobicity is best achieved with an HF concentration of from 0.1% to 0.5% by weight.
Furthermore, in order to achieve the above object, the result of polishing can be optimized if certain preferred conditions are observed during the polishing itself. Some of these conditions relate to apparatus features and some to process parameters. The apparatus features are explained in more detail below with reference to figures.